1. Field of the Invention
The present invention relates to a method and apparatus for transmitting video data.
2. Description of the Prior Art
In general, when a video signal (a television signal) is transmitted via a communication satellite, a frequency band of higher than 10 GHz is used. Radio waves of such a high frequency band are considerably attenuated by rainfall. In transmitting a video signal through a communication satellite, an FM modulation scheme is generally employed. However, if the signal level of the FM signal is attenuated to become lower than a threshold carrier-to-noise (C/N) ratio, the quality of the reproduced picture deteriorates rapidly. Accordingly, when the video signal is transmitted through the communication satellite, it is necessary to make up some suitable counterplan for bad weather such as rainfall or the like. In this connection, a serviceability of higher that 99.9% is requested in the case of bad weather.
In that case, if a direct broadcast by satellite (DBS) is performed between stationary stations, the image quality of the picture can be prevented from being deteriorated by increasing the transmission output. However, when the broadcasting station transmits a video image to its own station from a spot covered via a communication satellite, if the transmission output is increased, then not only the transmitter but also the power supply source apparatus or the like must be enlarged in size and increased in weight. Therefore, an increase of the transmission output is not desirable.
When, on the other hand, a video signal is digitized to perform the DBS, 4-phase shift keying (PSK) is frequently used as the modulation system. When the 4-phase PSK is employed, the attenuation of the signal by rainfall or the like can be avoided by switching it to a 2-phase PSK.
According to this method, however, the signal band is widened in the 2-phase PSK so that an expensive frequency band of the communication satellite must be increased. This is described in an article by Y. Yasuda et al., "THEORTICAL BIT ERROR RATE PERFORMANCE OF SOFT DECISION VITERBI DECODING,"Transaction of the Institute of Electronics Information and Communication Engineers, No. CS80-126, pp. 31 to 46, Sep. 24, 1980.
As described above, when the video signal is transmitted via the communications satellite, the image quality of the received picture is deteriorated by rainfall or the like. In order to remove this disadvantage, the prior art apparatus must be made large-sized, heavy in weight or complicated in construction. Alternatively, the signal band is widened.